1. Field of the Invention
The present invention generally relates to the art of optical communications, and more specifically to an electro-optic modulator configuration and method which enable modulation of an optical carrier with electrical signals having extremely high frequencies.
2. Description of the Related Art
Fiber optic links are becoming increasingly important in a wide variety of applications such as millimeter wave communications and radar systems. An external electro-optic modulator is usually required for a millimeter wave fiber optic link since direct modulation of a solid state laser signal is generally not possible above microwave frequencies.
Travelling wave integrated optic modulators used for this purpose are known in the art, such as described in a paper entitled "17 GHz bandwidth electro-optic modulator", by C. Gee et al, in Applied Physics Letters, vol. 43, no. 11, Dec. 1, 1983, pp. 998-1,000. A typical traveling wave modulator is illustrated in FIG. 1 and generally designated as 10. The modulator 10 includes a substrate 12 formed of an electro-optic material, preferably crystalline lithium niobate (LiNbO.sub.2) An optical waveguide 14 is formed in the substrate 12 just below the surface of the crystal by ion diffusion of titanium or proton exchange. The waveguide 14 is single mode, and typically only a few microns wide.
An optical signal from a laser or the like is fed into an input 16 and retrieved from an output 18 of the waveguide 14 using focussing lenses or by close coupling to single mode optical fibers (not shown). A microwave stripline electrode 20 including first and second segments 20a and 20b respectively is deposited on the surface of the substrate 12 immediately adjacent to the optical waveguide 14. An electrical signal at microwave or higher frequency is applied across the segments 20a and 20b through a coaxial cable 22. The electrode 20 is terminated in a resistive load 24 via a coaxial cable 26. The electrical signal applied to the electrode 20 through the cable 22 propagates along the electrode 20 parallel to the optical waveguide 14 as a traveling wave.
The segments 20a and 20b are sufficiently small and close together that the transverse electric field therebetween resulting from the electrical signal propagating along the electrode 20 passes through the optical waveguide 14 and induces an incremental phase shift in the optical signal via the electro-optic effect. This incremental phase shift is integrated along the length of the optical waveguide 14 to produce the net phase modulation. Although a phase modulator is illustrated in FIG. 1, the optical waveguide can be split into two branches in a Mach-Zehnder type interferometer arrangement to provide amplitude modulation as described in the above referenced article to Gee et al.
The integrated effect of the incremental phase shift is cumulative as long as the optical and electrical signals propagate parallel to each other at the same phase velocity. However, this does not occur in practical electro-optic materials such as LiNbO.sub.2. At optical frequencies, the refractive index of LiNbO.sub.2 is n.sub.o =2.2, whereas at microwave and millimeter wave frequencies the refractive index is n.sub.mm =5.3 to 6.6, depending on the orientation (LiNbO.sub.2 is anisotropic). Since the electric field between the segments 20a and 20b of the stripline electrode 20 passes through both air and LiNbO.sub.2, the effective index of refraction for the electrical signal travelling along the electrode 20 is on the order of n.sub.eff =4. This is still a mismatch with the n.sub.o =2.2 for the optical signal.
FIG. 2 illustrates the phase displacement of the electrical and optical signals as a function of distance of travel along the electrode 20. Due to the refractive index mismatch, the optical signal propagates with a phase velocity which is approximately twice that of the electrical signal. The magnitude of the phase modulation progressively decreases as the phase difference between the optical and electrical signals increases. This phenomenon is known as a phase "walk off". The decrease in overall phase modulation with frequency f and interaction length L is equal to [(sin(AfL))/AfL].sup.2, where A=2.pi./c(n.sub.eff -n.sub.o), and c is the speed of light.
This velocity mismatch necessitates design tradeoffs. The maximum achievable drive electrical drive signal frequency f decreases as the interaction length L is increased. Conversely, to lower the drive voltage and power, a long interaction length L is required. The modulator must be made shorter and the drive power larger as the frequency is increased to obtain satisfactory modulation.
Prior art attempts to compensate for this phase velocity mismatch include replacing the single electrode 20 with a periodic electrode structure such as described in a paper entitled "Velocity-matching techniques for integrated optic travelling wave switch/modulators", IEEE Journal of Quantum Electronics, vol. QE-20, no. 3, March 1984, pp. 301-309. These periodic electrode structures can be categorized into either periodic phase reversal or intermittent interaction electrodes. Known intermittent interaction electrode configurations include unbalanced transmission lines, i.e., asymmetric about the propagation axis. This leads to incompatibilities with the balanced line (typically coaxial or waveguide probe) transitions to other fiber optic link transmitter components.
The periodic phase reversal structures break up the electrode 20 into shorter sections, and force the phase shift between the sections to match the optical phase shift, as illustrated in FIG. 3. In the Figure, the electrode is assumed to consist of four sections, with a 180.degree. phase shift between the individual sections. The relative phase of the optical and electrical signals is effectively reset at the leading or upstream end of each section, and deviates to a maximum extent which is inversely proportional to the length of the sections. Thus, the phase velocities are matched on the average. However, there is still a reduction in the modulation by the factor [sin(AfL.sub.section)/AfL.sub.section ].sup.2, and L.sub.section is required to be long enough to produce a 180.degree. phase delay. This also means that the 180.degree. phase reversals are correct only at a single modulation frequency, so that the structure of FIG. 1, which is a low-pass modulator, is converted into a bandpass modulator.
Other problems that make it difficult to extend the operation of such modulators, both of conventional types such as shown in FIG. and the phase reversal types described above with reference to FIG. 3, to millimeter wave or higher frequencies, involve the connection of modulation electrodes to the modulation signal source by coaxial cables, or through wire bonds or the like. This becomes unmanageable due to the extremely small physical dimensions involved.